Is Landfill Waste Compatible with Geopolymer Matrix in Extreme Environments?

The implementation of Australia’s 2024 waste export ban has increased pressure on domestic recycling systems, resulting in an additional 650,000 tonnes of waste annually. This emphasises the urgent need for high volume landfill waste material recovery, especially in sustainable construction materials such as geopolymer concrete (GPC). Geopolymer concrete is recognised as a sustainable construction material; however, the scientific understanding of the compatibility between landfill waste and the geopolymer matrix, particularly under harsh environments, remains unknown. This paper presents an experimental investigation on five types of geopolymer concrete (GPC) mixes. The study included a control mix with natural stone chips and four additional mixes in which stone chips were 100% replaced with waste materials including shredded plastic, cardboard, crushed glass, and granular crumb rubber as fine aggregates. The mechanical performance, durability behaviour and stress-strain characteristics of these mixes were evaluated. Concrete samples were exposed to normal air, a saline environment with 10% salinity, and a hygrothermal environment at 60 °C and 98% humidity for four months to assess durability performance. The results demonstrate that GPC is compatible with landfill waste aggregates and enables the production of a workable mixture. As a result of saline environments, waste aggregate-based geopolymer concrete reduces compressive strength by 15%, while natural stone chips-based geopolymer concrete decreases strength by 45% during the same period, indicating that waste aggregates are more appropriate than natural aggregates in marine environments. Although the inclusion of waste aggregates reduces the strength and stiffness of the GPC, the materials continue to meet the mechanical property requirements for non-structural applications. A theoretical model considering the elastic modulus, ultimate strength and corresponding strain has been developed to predict compressive stress–strain behaviour of waste-based GPC. High modulus aggregates, typically ranging from approximately 10.0 GPa to 85.0 GPa such as stone chips and glass sand demonstrate parabolic stress–strain behaviour. In contrast low modulus aggregates, generally ranging from 1.0 GPa to 5.0 GPa including plastic, cardboard, and crumb rubber, exhibit a bilinear stress–strain response.